Level, angle and distance measuring device

ABSTRACT

The portable self-contained horizontal and vertical level, angle and distance measuring device is suitable for indicating whether or when two or more points are level with each other and for measuring distances and angles between two or more points, a point and a line or a point and a plane. Moreover, the measuring device is capable of displaying such measurements to a user in real time. The measuring device includes a measuring point ( 3 ) for identifying from where measurements are to be calculated, a user actuated trigger ( 11 ) and a display ( 9 ) for displaying measurements to the user either in real time or in memory mode. The measuring device is compact and light-weight making it particularly convenient and portable. It is also very versatile and is capable of replacing a multiplicity of measuring devices commonly employed in DIY, engineering, and in trade in general.

The present invention relates to a portable self-contained horizontaland vertical level, angle and distance measuring device (PMD) suitablefor indicating whether or when two or more points are level with eachother and for measuring distances and angles between two or more points,and preferably but not exclusively for displaying that indication and/ormeasurement to a user in real time.

As technology has moved on, a wide range of measuring tools have becomeavailable to the professional and do it yourself (DIY) tool user. Forhorizontal and vertical level indicators and measures there are spiritlevels, electronic levels and laser levels. For distance measurementthere are rulers, tape measures, ultrasonic and laser devices. For anglemeasurement there are protractors, angle finders, and T squares. Itwould not be unusual for a regular user of tools to require at leastfive of the aforementioned tools to be adequately equipped to undertakework.

The spirit level, and electronic level have physical size limitationswhich means that measurements beyond the length of the device aredifficult if not impossible to take accurately, and are limited to onedimension readings. A plumb line is awkward to use single handed, andhas to be set up to make long measurements. The laser level requires asurface to reflect off and has to be set up correctly, is oftenexpensive; and in addition, all laser devices have safety issues.

A straight-line ruler is limited by its length. A flexible tape, whilstlow cost and simple to use, is limited by potential variations inreading due to curves in the tape over long distances. The tape is proneto easily breaking, and injuring the user during rapid retraction of thetape into its housing. It is also particularly unwieldy to use over longdistances with just one operator.

While the ultrasonic distance measuring device is quick to use, small,and now relatively cheap to produce, it is limited by the distance itcan measure, and the requirement to bounce the emitted signal off aparallel surface. Its accuracy is affected by the air temperature anddensity, and it suffers from spurious readings due to additionalreflective noise. It can also only measure one co-ordinate at a time.

While the laser measuring system is also quick to use, small, andaccurate, it is currently expensive to produce, is adversely affected bybright ambient light e.g. sunshine, and is dependant on the surfacematerial of the measured object. It can also only measure oneco-ordinate at a time. Again, there are safety issues relating toproducts of this kind.

Protractors are impractical to use on large scale projects. Anglefinders have become increasingly sophisticated giving good accuracy, butcontinue to be limited by the physical constraints of their size. Tsquares are also cumbersome to carry around, and additionally arelimited to only giving 90 degree angles.

The present invention seeks to provide a new measuring device which issimple to use, portable, and obviates the need for most of theaforementioned tools, whilst at the same time offering three dimensionalmeasuring capabilities and freeing the user up from the limitingphysical constraints of the tools described previously.

The present invention thus provides a portable measuring devicecomprising: a housing; power supply means; a processor and one or moremotion sensors adapted to provide a measure of the relative spatialseparation of at least first and second locations; a user actuatedtrigger for identifying at least said first location; and a display forvisually presenting information on a measured relative spatialseparation characterised in that said one or more motion sensors detectmotion in six degrees of freedom and said processor is adapted todetermine at least one angle as a measure of said relative spatialseparation for presentation by said display.

In an alternative aspect the present invention provides a portablemeasuring device comprising: a housing; power supply means; a processorand one or more motion sensors adapted to provide a measure of therelative spatial separation of at least first and second locations; auser actuated trigger for identifying at least said first location; anda display for visually presenting information on a measured relativespatial separation characterised in that said processor is adapted todetermine at least one angle and a linear distance as a measure of saidrelative spatial separation for presentation by said display.

In a further aspect of the present invention there is provided aportable measuring device comprising: a housing; power supply means; aprocessor and one or more motion sensors adapted to provide a measure ofthe relative spatial separation of at least first and second locations;a user actuated trigger; and a display for visually presentinginformation on a measured relative spatial separation said measuringdevice being characterised by further including a measuring pointprovided on said housing having a defined spatial relationship withrespect to said one or more motion sensors, said measuring point beingprovided for identification to said processor, in association with saiduser actuated trigger, at least one of said first and second locations.

Many inertial measuring unit (IMU) based measurement systems rely on‘zero velocity updates’ (ZVUP) as described by C Verplaetse in “InertialProprioceptive devices: self-motion-sensing toys and tools”, IBM SystemsJournal, Vol. 35, Nos. 3&4 1996 and in U.S. Pat. No. 6,292,751. The‘zero velocity update’ relies on identifying when the sensors are atrest and resetting the velocity values back to zero. This processimproves the accuracy of an IMU system, and allows them to be used overa longer period of time without suffering ever increasing errors.However, such zero velocity updates are inherently impractical andinaccurate in a handheld device which is rarely completely stationary.

Preferably the present invention additionally encompasses non-zerovelocity updating of the measuring device in which the processor is incommunication with a volatile memory in which is stored calibration dataand a data store in which motion data is stored and the processor isadapted to update the calibration data and/or the stored motion data.When the measuring point is deemed substantially stationary, theprocessor can be adapted to determine an error correction to thecalibration data and/or the motion data in relation to motion detectedby said one or more motion sensors.

With the present invention the spatial separation in 3 axes ofmeasurement, between two points, or between a straight line and a point,or between a vertical or horizontal plane and a point, or between anyother flat plane and a point, can be measured and displayed. In apreferred embodiment acceleration, velocity, rotation and translationindications accounting for all six different degrees of freedom areprovided by means of electromechanical inertial measuring devices.Furthermore, the measuring device can determine the acceleration due togravity of the device and can compensate for this background signal.With the preferred embodiment the measuring device includes a series ofat least three accelerometers and three rate gyros to monitor movementof the measuring device in the six degrees of freedom. Ideally theaccelerometers and rate gyros are of MEMS (micro-electromechanicalsystems) technology to reduce the physical size requirements and tofacilitate a lightweight, low power consuming, hand-held device.

Reference herein to spatial separation is intended to encompassdifference in vertical height and horizontal separation, distance, angleto the horizontal or vertical, horizontal and vertical levelindications, etc.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIGS. 1 a, 1 b, 1 c and 1 d are outline drawings of a first embodimentof a portable measuring device in accordance with the present invention;

FIG. 2 is a diagram of a first embodiment of a measurement pointer foruse with a measuring device in accordance with the present invention;

FIG. 3 is a functional block diagram of a first embodiment of the pcbcontained in a portable measuring device in accordance with the presentinvention;

FIG. 4 is a flowchart describing an algorithm to determine when ameasuring device in accordance with the present invention is stationary;

FIG. 5 is a flowchart describing a method for re-calibrating astationary measuring device in accordance with the present invention;

FIG. 6 is a flowchart describing a method for re-calculating from storedtrajectory data the current position of a measuring device in accordancewith the present invention;

FIG. 7 is a flowchart describing an algorithm to determine a non-zerovelocity update (NZVUP) and the initial pitch and roll orientationangles for use with a measuring device in accordance with the presentinvention; and

FIGS. 8 a, 8 b, 8 c, 8 d, 8 e, 8 f, 8 g, 8 h are examples of displayscreens of a measuring device in accordance with the present invention.

To assist in an understanding of how spatial separation is measuredusing an inertial measuring unit (IMU) the following is an explanationof its basic principles.

Spatial separation is measured by monitoring movement of the IMU. Themovement of the IML is detected by sensors measuring motion in sixdegrees of freedom i.e. three rotational and three translationalmovements, relating to movement of a reference point associated with theIMU, and typically within it. Typically these sensors comprise threerate gyros and three accelerometers organised and aligned in three axes,each perpendicular to the other two axes.

These sensors measure motion with respect to a co-ordinate frame fixedto the IMU (referred to as the “body frame”). These measured motionsneed to be translated into motion in a co-ordinate frame aligned to theforce of gravity i.e. one axis parallel to the gravitational force(vertical) and the other two axes perpendicular to the gravitationalforce (horizontal) and each other (referred to as the “local frame”) inorder to allow for the gravitational effects on the measured motion ofthe IMU. It is well understood that motion with respect to the bodyframe can be translated into motion with respect to the local frameusing a direction cosine matrix, derived from Euler angles or EulerParameters (quaternions).

In a typical embodiment of a measuring device in accordance with thepresent invention a spatial separation is measured by the followingprocess:

-   -   The accelerometer and rate gyro sensors of the IMU are sampled        at regular intervals, for example every 1 ms    -   The start of measurement is indicated by a user actuated        trigger, indicating, at that moment in time, that the measuring        device is stationary i.e. with zero velocity and zero        translational acceleration.    -   The initial orientation of the measuring device with respect to        the local frame, e.g. pitch, roll and yaw, is calculated by        resolving the three components of gravitational acceleration        with respect to the body frame, as measured by the        accelerometers, using simple trigonometry. The initial position        co-ordinates of the measuring point with respect to the local        frame are set to zero.    -   During subsequent movement of the measuring device, measurement        of the orientation and translational movement of the measuring        point of the measuring device are re-calculated for each sample        of the sensor values of the IMU e.g. each millisecond, using the        following algorithm:    -   The angular velocities with respect to the body frame are        calculated from the values measured by the rate gyro sensors of        the IMU.    -   The quaternions are calculated from the angular velocities with        respect to the body frame and the initial orientation of the        measuring device with respect to the local frame.    -   The direction cosine matrix is calculated from the quaternions.    -   The gravitational acceleration component with respect to the        local frame is in the vertical axis only (by definition).    -   The gravitational acceleration components of the IMU with        respect to the body frame are calculated from the vertical        gravitational acceleration component with respect to the local        frame and the direction cosine matrix.    -   The total accelerations of the IMU with respect to the body        frame are calculated from the values measured by the        accelerometer sensors of the IMU.    -   The translational accelerations of the IMU with respect to the        body frame are calculated by subtracting the gravitational        acceleration components with respect to the body frame from the        total accelerations of the IMU with respect to the body frame.    -   The translational velocity of the IMU with respect to the body        frame is calculated by integrating the translational        accelerations of the IMU with respect to the body frame.    -   The translational velocity of the IMU with respect to the local        frame is calculated from the translational velocity of the IMU        with respect to the body frame and the direction cosine matrix.    -   The translational movement of the IMU with respect to the local        frame is calculated by integrating the translational velocity of        the IMU with respect to the local frame.    -   The angular orientation of the measuring device with respect to        the local frame is calculated from the quaternions.    -   The translational movement of the measuring point with respect        to the local frame is calculated from the translational movement        of the IMU with respect to the local frame, the angular        orientation of the measuring device with respect to the local        frame, and the fixed spatial relationship between the IMU and        the measuring point.    -   The relative spatial separation of the measuring point from its        initial position with respect to the local frame, is calculated        from the translational movement with respect to the local frame,        using simple trigonometry.    -   All measurements that are then displayed by the measuring device        e.g. difference in vertical height and/or horizontal separation,        distance, angle to the horizontal or vertical, horizontal and        vertical level indications, etc. are calculated from the        relative spatial separation with respect to the local frame.

A portable measuring device (PMD) is shown in FIGS. 1 a, 1 b, 1 c and 1d consisting of a housing 1 in the interior of which is located aplurality of motion sensors in the form of inertial measurementcomponents as a self-contained unit. The inertial measurementcomponents, preferably in the form of an inertial measuring unit (IMU)2, and their associated electronic interface components are typicallyprone to drift due to temperature variation. In use, the PMD may besubject to rapid temperature variations e.g. heat from a user's hand. Tominimise the effect of a variation in external temperature on theinternal components of the PMD, the material of the housing 1 ispreferably selected to be thermally insulating and thus have a highthermal resistance. Also, the housing 1 may be sealed to eliminatevariations in internal temperature due to convection.

A measuring point 3 is provided on the exterior of the housing 1 againstwhich all spatial measurements of the PMD are referenced. The measuringpoint 3 may be an integral part of the housing 1 or may be connectedthereto and is visually distinguishable and capable of alignment by auser with a selected location from or to which measurements are to betaken. Additionally virtual measuring points representing locationsremote from the PMD may be identified by the PMD by means of a laserbeam or other beam.

The housing 1 of the PMD also includes a trigger 4 which is useractuated for example, but not limited to, manually, mechanically,electronically or by voice. The trigger 4 is connected to a microswitch5 mounted internally of the housing on a pcb 6. Preferably, the trigger4 is in close proximity to the microswitch 5 such that each time thetrigger 4 is depressed, the trigger 4 activates the microswitch 5 whichin turn supplies a signal to a processor 7 also mounted on the pcb 6.

When the PMD is placed against a solid surface to make a measurement,the deceleration force exerted on the IMU 2 may typically be in theorder of 10's or 100's of g, and therefore in excess of the measuringrange of the IMU 2. To enable the IMU 2 to be able to measure such adeceleration force, a deceleration device 8 (FIG. 2) is additionallyprovided on the housing 1 of the PMD, mounted in a configuration suchthat in normal operation of the PMD, the deceleration device 8 is thefirst component of the PMD to make contact with a location point from orto which measurements are to be made. The deceleration device 8 ispreferably compressible and thus provides a means to limit thedeceleration force on the IMU 2 to within its measurement range. In thisrespect the deceleration device may include a compressible material or acompressible element such as a spring. The deceleration device 8 may bean integral part of the housing 1 or may be connected thereto. Asillustrated in FIG. 2 in a preferred embodiment of the measuring devicethe deceleration device 8 and the trigger 4 are combined such thatcompression of the deceleration device 8 actuates the trigger 4.Alternatively, the trigger 4 can be actuated by the user moving the PMDin a pre-defined manner so as to expose the IMU 2 to a particularpattern of acceleration forces. Of course the deceleration device 8 maybe omitted either where the IMU 2 has a measuring range whichencompasses the deceleration forces likely to be encountered or wheresuch large deceleration forces do not need to be controlled.

The housing 1 also includes a transparent window 9 aligned with adisplay 10 mounted on the pcb 6. Alternatively, the display 10 may formpart of the housing 1. One or more switches in the form of push buttons11 are provided on the housing 1 (three are illustrated in FIGS. 1 a and1 b). The push buttons 11 enable a user to control the operation of thePMD. The push buttons 11 are either connected to or mounted in closeproximity to control switches 12 on the pcb 6, such that a respectivecontrol switch 12 is activated when a user depresses its associated pushbutton 11.

Turning now to FIG. 3 as mentioned above, a pcb 6 is used to mount andconnect the internal components of the PMD. The internal componentsinclude the inertial measuring unit (IMU) 2 which is used to provide toa processor 7 electrical signals relating to the translational androtational movement and orientation of a reference point associated withthe IMU 2. The processor 7 is programmed with the fixed spatialrelationship between the reference point of the IMU 2 and the measuringpoint 3 on the housing 1 so that the translational and rotationalmovement and orientation of the measuring point 3 can be determined bythe processor 7. Alternatively, each individual inertial measurementcomponent in the IMU may have its own reference point, in which case theprocessor is programmed with a series of relationships for the spatialdifference between the measuring point 3 and each of the individualreference points. A special case of this is where the reference point(s)are coincident with said measuring point 3.

In an alternative embodiment (not illustrated) a laser emitter anddetector is provided in or connected to the PMD to enable non-contactrelative measurements to be performed by means of conventional capturelaser distance measurement techniques such as those described in U.S.Pat. No. 6,191,845. Each remote point of reflection of the laser beam istreated by the IMU 2 as a virtual measuring point and, as the laser beamis deemed to travel in a straight line, the relative spatial separationof different points of reflection can be determined by the PMD usingconventional trigonometric theory. With this embodiment it is not thetranslational and rotational movement of the virtual measuring pointthat is determined but rather measurement of the translational androtational movement of the PMD, relative to the virtual measuringpoints, which enables measurement of the spatial separation of the twolocations.

Typically, the IMU 2 contains a plurality of accelerometers and rategyros, preferably mounted in the x, y and z axes, which provideelectrical signals to the processor 7, proportional to the translationalacceleration and rotational velocity of the IMU 2. Alternativeembodiments of the IMU 2 may include any combination of motion sensorsincluding but not limited to force measuring devices such astranslational and angular accelerometers, rate gyros and magnetic fielddetectors such as magnetometers. Alternative arrangements of the motionsensors, for example in a pyramidal structure, are also envisaged.Whilst the IML 2 is described as being mounted on the pcb 6, alternativeconfigurations are envisaged which would require additional pcbs orindeed obviate the need for any pcbs. To provide a compact andlightweight structure the IMU 2 is preferably fabricated using MEMStechnology such as that described in U.S. Pat. No. 6,456,939, and U.S.Pat. No. 6,295,870 and patent application US2002/0065626.

The IMU 2 may be provided with an additional accelerometer 20, formeasuring significantly higher deceleration forces in one axis only.This axis is aligned within the PMD to measure decelerations in thedirection of normal motion of the PMD and/or the deceleration device 8as a user places it against a surface location to be measured. Thisadditional accelerometer 20 may be external to the IMU 2.

A temperature sensor 13 is provided and is connected to the processor 7.The temperature sensor 13 outputs a signal to the processor 7proportional to the internal temperature of the PMD. This is used toenable the processor 7 to provide temperature compensation for thesignals received from the IMU 2, which are typically temperaturedependent. Although illustrated separate from the IMU 2, temperaturesensors may be incorporated into one or more of the individual inertialmeasurement components of the IMU 2 to provide more accurate temperaturecompensation.

The microswitch 5 which is mounted on the pcb 6 and is activated by thetrigger 4 is connected to the processor 7 and outputs an electricaltrigger signal to the processor 7 each time the trigger 4 is manuallyactivated by a user or mechanically activated by positioning the trigger4, or a component such as the deceleration device 8, against a surface.

The processor 7 is also connected to a memory 14. The memory 14 includes3 allotted memory regions, a first memory region 14 a in which thecalibration data for the IMU 2 is stored, a second memory region 14 b inwhich reference location data is stored, and a third memory region 14 cin which the trajectory data may be stored. The calibration data for theIMU 2 stored in the first region 14 a of the memory may bepredetermined. Alternatively the calibration data for the IMU 2 may beobtained during the normal operation of the PMD, and stored in thememory 14 a. A further program memory may be associated with theprocessor in which the instructions and algorithms for calculating therelative spatial separation between first and second locations arestored.

A display 10 is connected to the processor 7 and is used to continuouslydisplay real-time data supplied by the processor 7 on the relativespatial separation of the measuring point of the PMD from a previouslystored reference location in space which was stored in the referencedata memory region 14 b.

A power source 15, preferably in the form of a battery, is connected viapower supply means to the internal electrical and electromechanicalcomponents to supply power for the electronic and electromechanicalcomponents. Alternative power sources such as solar cells are alsosuitable for powering the measuring device.

A clock 16 is connected to the processor 7, and provides a clockingsignal to the processor 7, to enable the processor to take successivemeasurements from the IU 2 at predetermined regular time intervals, e.g.1 mS to 1000 mS. Additionally the processor has access to or includesthe functionality of a timer to monitor the time taken for a measurementto be taken. Ideally, the timer is in communication with the clock 16and consists of an incremental counter 17 which counts the number ofclock pulses issued during the taking of a measurement. In this way thenumber of clock pulses counted is representative of the duration of themeasurement. Each time the trigger 4 is actuated to identify a new‘start point’ the counter 17 is preferably re-set to zero. Furthermore,the processor 7 may use the information from the counter 17 to determinean appropriate resolution for the measurement being taken. In this waythe resolution of the measurement may be varied in dependence upon thetime taken for the measurement which in turn in general may reflect thescale of the measurement (e.g. millimetres, centimetres or metres) orthe accessibility of the second location, for example.

One or more control switches 12 are also mounted on the pcb 6 and areconnected to the processor 7. The control switches 12 are used to enablea user to select the operation of the processor 7 from a predeterminedset of functions, and each control switch 12 supplies a signal to theprocessor 7 each time the associated push button 11 is pressed by auser. The control switches 12 may be used for example to select whethera measurement is to be made from a point, a line or a plane; theengineering units used to display the measurement e.g. millimetres andcentimetres and metres, feet and inches, degrees or angular ratios; thetype of measurement to be taken e.g. a first location or secondlocation; how the measurement is to be displayed e.g. as a level or as adistance, angle, area or volume. An audible sounder 18 may be providedand connected to the processor 7, and used to provide audible feedbackto a user during operation of the PMD.

A port 19 may be provided and connected to the processor 7, and is usedto extract the data stored by the processor 7 for further analysis.

When in use, the processor 7 receives signals from the IMU 2,corresponding to the translational and rotational movement andorientation of the reference point of the IMU 2, including translationalaccelerations, rotational velocities, velocity increments, positionalincrements and angular increments or to the relative positional androtational movement and orientation of the reference point of the IMU 2with respect to an earlier position. It is also envisaged that an IMUmight be employed that uses components for providing some but not all ofthe measurements listed above.

The processor 7 receives an indication from a user by means of one ofthe control switches 12 that the start of a measurement is to be taken,indicating that the measuring point 3 on the PMD is at the firstlocation referred to as the ‘start point’, and that the user is holdingthe device substantially stationary. The processor 7 is programmed withpre-defined limits of human hand and body movements, and compares theselimits with the acceleration and angular rate parameter values receivedfrom the IMU 2 to determine when the PMD is substantially stationary.The flow chart in FIG. 4 details an algorithm (S1-S4) that may be usedto determine that the PMD is stationary. The processor 7 resets thetranslational and rotational velocity parameter values to zero,determines the orientation of the PMD relative to the vertical, andresets the position co-ordinates of the measuring point 3 on the PMD tozero. The processor 7 stores all the parameter values generated by orderived from the IMU 2 as a data set into the ‘start point’ location ofthe reference location data memory region 14 b.

In an alternative embodiment, the processor 7 waits for the trigger 4 tobe activated, indicating that the measuring point 3 is known to bestationary, for example while it is held against a solid object. Underthese circumstances a relationship is established between the measuringpoint 3 which is at zero velocity, and the reference point of the IMU 2which is experiencing movement due to the hand movement of the personholding the PMD. The processor 7 carries out a ‘non-zero velocityupdate’ (NZVUP) to determine the initial velocity of the reference pointof the IMU and initial orientation of the PMD to the vertical. Theflowchart in FIG. 7 (S21-29) describes an algorithm for performing anon-zero velocity and orientation calibration update. The processor 7stores all the parameter values generated by or derived from the IMU 2as a data set into the ‘start point’ location of the reference locationdata memory region 14 b.

The processor 7 may then activate the audible sounder 18 to inform auser that the ‘start point’ measurement is complete, and the device canbe moved.

As a user moves the PMD, the processor 7 receives new parameter valuesfrom the IMU 2. The processor 7 uses these parameter values, togetherwith the corresponding parameter values stored in the ‘start point’location of the reference location data memory region 14 b, and theknown spatial relationship between the measuring point 3 and thereference point of the IMU 2, to derive a spatial separation in terms ofa three dimensional spatial difference measurement between the currentposition of the measuring point 3 and its position at the ‘start point’.The processor 7 may in addition derive the difference in both a verticalplane and a horizontal plane between the current and the ‘start point’positions of the measuring point 3.

The processor 7 displays the difference measurement to a user on thedisplay 10 in real time such that a user is provided with a continuousand substantially instantaneous display indicating the differencemeasurement of the measuring point 3 from the ‘start point’. In thisrespect the second location for which a relative measurement from thefirst location, or ‘start point’, is required is treated as theinstantaneous position of the PMD. However, in an alternative embodimentthe trigger 4 may be used to identify for the processor 7 the secondlocation for which a measurement is required in which case acontinuously updated real time display is not necessary and instead themeasurement is displayed only after the trigger 4 has been actuated toidentify the second location. Of course the measurements can bedisplayed to a user in a number of different formats.

As mentioned earlier, the PMD may also be used to derive and displaydifference measurements relative to a reference line or a referenceplane. Via the control switches 12 a user is able to instruct theprocessor 7 that additional reference points are to be captured afterthe first ‘start point’ measurement has been captured to define areference line or reference plane. The processor 7 takes a measurementusing the same method as for the ‘start point’ location, but stores theparameter values generated by and derived from the IMU 2 into asecondary location of the reference location data memory region 14 b.The ‘start point’ location and secondary reference point location can beused by the processor 7 to define a reference line relative to the‘start point’, and subsequent difference measurements may be derived anddisplayed relative to the ‘start point’ on this reference line. In asimilar manner, a third reference point can be captured, to define areference plane relative to the ‘start point’, and the processor 7 canderive and display the difference measurements relative to the ‘startpoint’ on this reference plane.

The PMD is also particularly suited to function as an electronic level.In this respect part of the inherent functions of the PMD is themeasurement of angular relationships between points and points, lines orplanes. A level simply identifies in respect to a horizontal or verticalplane, when there is no angular difference between two points withrespect to the horizontal or vertical plane i.e. the measured angle is0°. The PMD may display this information by displaying a real-timeangular difference measurement or the display may alternatively oradditionally provide a graphical indication of level, as illustrated inFIGS. 8 a and 8 b.

When all measurements are complete the data held in the memory 14 may berecalled by a user on the display 10, or downloaded via the port 19 intoa computer for subsequent analysis and/or display.

The signals produced by the IMU 2 are prone to drift with both time andtemperature which, due to the calculations for translational and angularmovement carried out by the processor 7, increases measurement errorswith time. To minimise these errors, the processor 7 may adjust thecalibration data for each of the sensing elements contained within theIMU 2 stored in the calibration data memory region 14a. Alternatively oradditionally, the processor 7 may apply a correction factor to theindividual signals received from the IMU 2 or to the calculated relativetranslational and rotational movement of the measuring point asdetermined by the processor. One or more means for adjusting thecalibration data or measurement signals during normal use of the PMD maybe provided in the PMD.

As mentioned earlier, the processor 7 uses the signal from thetemperature sensor 13 to determine the internal temperature of the PMDand hence the temperature of the components of the IMU 2. The processor7 is programmed with a series of temperature related correction factorsfor each component of the IMU 2, and the processor 7 uses thesecorrection factors to adjust the calibration data for each of thecomponents of the IMU 2 stored in the calibration data memory region 14a at regular time intervals e.g. 1 second to 60 seconds. Alternativelythe processor 7 may use the temperature related correction factors toadjust each instantaneous measurement signal received from the IMU 2.

Also, whenever the measuring point 3 on the PMD is determined to bestationary during a ‘start point’ measurement, certain parameter valuesgenerated by, or derived from, the IMU 2 can be corrected, for examplethe orientation of the PMD, and/or to compensate for localisedenvironmental conditions, and the processor 7 can adjust values for the‘start point’ calibration data set stored in the calibration data memoryregion 14 a to remove any offsets for those signals. The flowchart inFIG. 5 (S5-S11) describes an example of an algorithm for thisre-calibration.

Also, whenever the measuring point 3 on the PMD is determined to bestationary other than during a ‘start point’ measurement, the processor7 may similarly derive new calibration data for each of the sensingelements of the IMU 2 and store them as a new calibration data set inthe next available location in the calibration data memory region 14 a.The processor 7 may also carry out a non-zero velocity update (NZVUP) atleast to separately calculate the current velocity of the referencepoint of the IMU and orientation of the PMD, in order to correct thecurrent values for these parameters. The processor 7 may also derive theaccumulated error values for these parameters, and store them as part ofthe new calibration data set in the calibration data memory region 14 a.

At the same time as the processor 7 derives and displays the differencemeasurements, the parameter values generated by, and derived from, theIMU 2 may also be recorded, as a data set, by the processor 7 into thetrajectory data memory region 14 c at regular time intervals.

The processor 7 uses the sets of calibration data stored in thecalibration data memory region 14 a, and the sets of parameter valuesrecorded in the trajectory data memory region 14 c, and by means ofinterpolation between adjacent sets of calibration data, derives a newset of calibration data associated with each set of parameter values,and then uses each new set of calibration data and each set of parametervalues to re-calculate the spatial separation of the measuring point 3on the PMD from the ‘start point’ to its current position.

The processor 7 uses this revised spatial separation and displays it toa user on the display 10. The flow chart in FIG. 6 (S12-S20) describesan algorithm for this re-calculation.

The trajectory data memory region 14 c and the calibration data memoryregion 14 a are typically over-written with new data each time a new‘start point’ measurement is taken.

The nature of the calculations carried out by the processor 7 means thaterrors in the measurement can accumulate with time. To partiallycompensate for these errors, the processor 7 can adjust the resolutionof the values displayed on the display 10 in relation to the elapsedduration of the measurement. The longer the duration of the measurement,the lower the resolution displayed.

By incorporating laser distance measurement into the PMD the possibilityof measuring to points that cannot be reached by the user of the PMDsuch as a tall roof become possible. As mentioned earlier, where lasermeasurement is combined with inertial measurement the positions of tworemote locations which are inaccessible to the user can be captured andthe spatial separation of such remote locations can then be determinedusing trigonometric theory and the monitored inertial movement of thePMD between the capture of the first and second of the remote locations.Of course, the full functionality of the PMD in terms of measuringrelative levels, and angles and distances relative to lines or planesalso applies. Moreover, measurements between virtual measuring pointsgenerated by means of the laser emitter and locations identified usingthe measuring point 3 provided on the PMD housing are also possible.Although reference is made herein to a laser emitter and detector itwill be apparent that alternative forms of non-contact distance measuresmay be employed including ultrasonic devices. By incorporating anon-contact meter in the PMD the possibility arises to utilise the PMDto measure around corners, around objects, over busy roads or widerivers and to capture a plurality of measurements quickly and easilywith a minimum of movement.

Further adaptations and alterations of the PMD are envisaged withoutdeparting from the scope of the invention defined in the appendedclaims.

1. A portable measuring device comprising: a housing; power supplymeans; a processor and one or more motion sensors adapted to provide ameasure of the relative spatial separation of at least first and secondlocations; a user actuated trigger for identifying at least said firstlocation; and a display for visually presenting information on ameasured relative spatial separation wherein said one or more motionsensors detect motion in six degrees of freedom and said processor isadapted to determine at least one angle as a measure of said relativespatial separation for presentation by said display.
 2. The portablemeasuring device as claimed in claim 1, wherein said processor isadapted to determine said at least one angle with respect to one or bothof vertical and horizontal planes.
 3. The portable measuring device asclaimed in claim 2, wherein said processor is adapted to determinewhether said first and second locations are level with respect to eitherof said vertical or horizontal planes.
 4. The portable measuring deviceas claimed in claim 1, wherein said processor is adapted to determine,in addition to said at least one angle, a linear distance separatingsaid first and second locations.
 5. A portable measuring devicecomprising: a housing; power supply means; a processor and one or moremotion sensors adapted to provide a measure of the relative spatialseparation of at least first and second locations; a user actuatedtrigger for identifying at least said first location; and a display forvisually presenting information on a measured relative spatialseparation wherein said processor is adapted to determine at least oneangle and a linear distance as a measure of said relative spatialseparation for presentation by said display.
 6. A portable measuringdevice comprising: a housing; power supply means; a processor and one ormore motion sensors adapted to provide a measure of the relative spatialseparation of at least first and second locations; a user actuatedtrigger; and a display for visually presenting information on a measuredrelative spatial separation said measuring device further including ameasuring point provided on said housing having a defined spatialrelationship with respect to said one or more motion sensors, saidmeasuring point being provided for identification to said processor, inassociation with said user actuated trigger, at least one of said firstand second locations.
 7. The portable measuring device as claimed inclaim 6, wherein said measuring point is visually distinguishable onsaid housing and user alignable with a user selected spatial location.8. The portable measuring device as claimed in claim 6, wherein saidmeasuring point is adapted to be substantially stationary when alignedby a user with a selected spatial location.
 9. The portable measuringdevice as claimed in claim 8, wherein processor is adapted to determinean error correction when said measuring point is aligned with a selectedspatial location and is substantially stationary, in relation to motiondetected by said one or more motion sensors.
 10. (canceled)
 11. Theportable measuring device as claimed in claim 6, wherein the processoris-in communication with-a volatile memory in which is storedcalibration data and the processor is adapted to update calibration datastored in said volatile memory at a second or subsequent location. 12.(canceled)
 13. The portable measuring device as claimed in claim 11,wherein said processor is adapted to adjust for movement of the one ormore motion sensors as a result of uncontrolled hand movements of theuser when updating calibration data stored in said volatile memory. 14.(canceled)
 15. A portable measuring device as claimed in claim 6,comprising a plurality of motion sensors consisting of at least threeaccelerometers and three angular rate sensors.
 16. (canceled) 17.(canceled)
 18. The portable measuring device as claimed in claim 6,further including a timer, in communication with the processor, formonitoring the time duration of a measurement wherein the processor isadapted to determine the measure of relative spatial separation to aresolution dependent upon the time duration of the measurement.
 19. Theportable measuring device as claimed in claim 6, wherein the processoris adapted to determine from information received from the motionsensors when the measuring device is stationary and to generate an errorcorrection.
 20. The portable measuring device as claimed in claim 6,wherein the processor has access to threshold data identifying lowerlimits of measurable spatial movement representative of small,uncontrolled hand movements of a user.
 21. The portable measuring deviceas claimed in claim 6, further comprising a deceleration device forreducing high deceleration forces. 22-25. (canceled)
 26. The portablemeasuring device as claimed in claim 6, wherein the processor is adaptedto supply real time data on the measured relative spatial separation.27. The portable measuring device as claimed in claim 6, wherein saidfirst location, from which the spatial separation of said secondlocation is determined, is selected from a reference point, a referenceline or a reference plane.
 28. The portable measuring device as claimedin claim 6, wherein the processor additionally includes a data store inwhich motion data is stored and said processor is adapted to update saidstored motion data in dependence on calculated error corrections orupdated calibration data and to recalculate said measured spatialseparation in dependence on the updated motion data.
 29. (canceled) 30.The portable measuring device as claimed in claim 6, further including anon-contact distance meter for measuring a distance to a position remotefrom the measuring device, the position being at least one of said firstand second locations.